Not legal advice, no atty-client relationship express or implied . . .
There is simply not enough information to determine whether the car has been legally sold. It depends on the terms and conditions that were exchanged between the prospective buyer and seller as well as the conduct of each in the transaction.
Generally speaking, the UCC will apply in either state. Here are a few sections that may apply to this transaction.
2-204 Formation in general.
(1) A contract for sale of goods may be made in any manner sufficient to show agreement, including conduct by both parties which recognizes the existence of such a contract.
(2) An agreement sufficient to constitute a contract for sale may be found even though the moment of its making is undetermined.
(3) Even though one or more terms are left open a contract for sale does not fail for indefiniteness if the parties have intended to make a contract and there is a reasonably certain basis for giving an appropriate remedy.
2-206 Offer and acceptance in formation of contract.
(1) Unless otherwise unambiguously indicated by the language or circumstances:
(a) An offer to make a contract shall be construed as inviting acceptance in any manner and by any medium reasonable in the circumstances;
(b) An order or other offer to buy goods for prompt or current shipment shall be construed as inviting acceptance either by a prompt promise to ship or by the prompt or current shipment of conforming or nonconforming goods, but such a shipment of nonconforming goods does not constitute an acceptance if the seller seasonably notifies the buyer that the shipment is offered only as an accommodation to the buyer.
(2) Where the beginning of a requested performance is a reasonable mode of acceptance an offeror who is not notified of acceptance within a reasonable time may treat the offer as having lapsed before acceptance.
2-207 Additional terms in acceptance or confirmation.
(1) A definite and seasonable expression of acceptance or a written confirmation which is sent within a reasonable time operates as an acceptance even though it states terms additional to or different from those offered or agreed upon, unless acceptance is expressly made conditional on assent to the additional or different terms.
(2) The additional terms are to be construed as proposals for addition to the contract. Between merchants such terms become part of the contract unless:
(a) The offer expressly limits acceptance to the terms of the offer;
(b) They materially alter it; or
(c) Notification of objection to them has already been given or is given within a reasonable time after notice of them is received.
(3) Conduct by both parties which recognizes the existence of a contract is sufficient to establish a contract for sale although the writings of the parties do not otherwise establish a contract. In such case the terms of the particular contract consist of those terms on which the writings of the parties agree, together with any supplementary terms incorporated under any other provisions of chs. 401 to 411. [In WI, other states had different statute numbers for their adoption fo the UCC]
2-401 Passing of title; reservation for security; limited application of
(1) Title to goods cannot pass under a contract for sale prior to their identification to the contract (2-501), and unless otherwise explicitly agreed the buyer acquires by their identification a special property as limited by chs. 401 to 411. Any retention or reservation by the seller of the title (property) in goods shipped or delivered to the buyer is limited in effect to a reservation of a security interest. Subject to these provisions and to ch. 409, title to goods passes from the seller to the buyer in any manner and on any conditions explicitly agreed on by the parties.
(2) Unless otherwise explicitly agreed title passes to the buyer at the time and place at which the seller completes the seller's performance with reference to the physical delivery of the goods, despite any reservation of a security interest and even though a document of title is to be delivered at a different time or place; and in particular and despite any reservation of a security interest by the bill of lading:
(a) If the contract requires or authorizes the seller to send the goods to the buyer but does not require the seller to deliver them at destination, title passes to the buyer at the time and place of shipment; but
(b) If the contract requires delivery at destination, title passes on tender there.
(3) Unless otherwise explicitly agreed where delivery is to be made without moving the goods:
(a) If the seller is to deliver a document of title, title passes at the time when and the place where the seller delivers such documents; or
(b) If the goods are at the time of contracting already identified and no documents are to be delivered, title passes at the time and place of contracting.
(4) A rejection or other refusal by the buyer to receive or retain the goods, whether or not justified, or a justified revocation of acceptance revests title to the goods in the seller. Such revesting occurs by operation of law and is not a "sale".
2-502 Buyer's right to goods on seller's repudiation, failure to deliver, or insolvency.
(1) Subject to subs. (2) and (3) and even though the goods have not been shipped a buyer who has paid a part or all of the price of goods in which the buyer has a special property under 2-501may on making and keeping good a tender of any unpaid portion of their price recover them from the seller if:
(a) In the case of goods bought for personal, family, or household purposes, the seller repudiates or fails to deliver as required by the contract; or
(b) In all cases, the seller becomes insolvent within 10 days after receipt of the first installment on their price.
(2) The buyer's right to recover the goods under sub. (1) (a) vests upon acquisition of a special property, even if the seller had not then repudiated or failed to deliver.
(3) If the identification creating the buyer's special property has been made by the buyer, the buyer acquires the right to recover the goods only if they conform to the contract for sale.
2-503 Manner of seller's tender of delivery.
(1) Tender of delivery requires that the seller put and hold conforming goods at the buyer's disposition and give the buyer any notification reasonably necessary to enable the buyer to take delivery. The manner, time and place for tender are determined by the agreement and this chapter, and in particular:
(a) Tender must be at a reasonable hour, and if it is of goods they must be kept available for the period reasonably necessary to enable the buyer to take possession; but
(b) Unless otherwise agreed the buyer must furnish facilities reasonably suited to the receipt of the goods.
(2) Where the case is within 2-504 respecting shipment tender requires that the seller comply with its provisions.
(3) Where the seller is required to deliver at a particular destination tender requires that the seller comply with sub. (1) and also in any appropriate case tender documents as described in subs. (4) and (5).
(4) Where goods are in the possession of a bailee and are to be delivered without being moved:
(a) Tender requires that the seller either tender a negotiable document of title covering such goods or procure acknowledgment by the bailee of the buyer's right to possession of the goods; but
(b) Tender to the buyer of a nonnegotiable document of title or of a written direction to the bailee to deliver is sufficient tender unless the buyer seasonably objects, and receipt by the bailee of notification of the buyer's rights fixes those rights as against the bailee and all third persons; but risk of loss of the goods and of any failure by the bailee to honor the nonnegotiable document of title or to obey the direction remains on the seller until the buyer has had a reasonable time to present the document or direction, and a refusal by the bailee to honor the document or to obey the direction defeats the tender.
(5) Where the contract requires the seller to deliver documents:
(a) The seller must tender all such documents in correct form, except as provided in s. 2-323(2) with respect to bills of lading in a set; and
(b) Tender through customary banking channels is sufficient and dishonor of a draft accompanying the documents constitutes nonacceptance or rejection.
2-504 Shipment by seller.
(1) Where the seller is required or authorized to send the goods to the buyer and the contract does not require the seller to deliver them at a particular destination, then unless otherwise agreed the seller must:
(a) Put the goods in the possession of such a carrier and make such a contract for their transportation as may be reasonable having regard to the nature of the goods and other circumstances of the case; and
(b) Obtain and promptly deliver or tender in due form any document necessary to enable the buyer to obtain possession of the goods or otherwise required by the agreement or by usage of trade; and
(c) Promptly notify the buyer of the shipment.
(2) Failure to notify the buyer under sub. (1) (c) or to make a proper contract under sub. (1) (a) is a ground for rejection only if material delay or loss ensues.
There are many other provisions of the UCC that may apply, as well as statutes and case law for each individual state that may also be relevant.
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In any event, the practical reality of the expense of litigation, particularly when multiple states are involved, is such that a lawsuit is unlikely to develop from this mishap.
From a strictly practical perspective, and since your prospective buyer likely knows of this situation (or will know of it shortly) given his relationship to KGP, my personal advice is to be very apologetic and honest and offer to compensate him for any out-of-pocket expenses that he incurred as a result of your cold feet.
I am not going to lecture you, but don't lose sight of the fact that this is your mistake and you need to do the right thing to fix it. The right thing is to make the prospective buyer whole, i.e. in the same position he was in before he found your ad.